Many studies have evaluated the chemical vapor deposition (CVD) of tungsten (W) used in semiconductor device manufacturing. One of the most common chemical schemes used in the CVD of tungsten in semiconductor device manufacture involves deposition of tungsten from tungsten hexafluoride (WF.sub.6) and hydrogen (H.sub.2) gases in a process referred to as the hydrogen reduction process. Typically the hydrogen reduction process involves a premixing H.sub.2 and WF.sub.6 gases at a sub reaction temperature in an inlet region of a reactor and then directing the gases onto the surface of a wafer to be coated, which is maintained at an elevated reaction temperature of, for example, 450.degree. C. When the mixed gases contact the wafer at this temperature, the WF.sub.6 and H.sub.2 gases react to produce elemental tungsten, which is deposited onto the wafer as a film, and HF, which is exhausted from the reactor.
The initial stage of growth of the film is referred to as "nucleation", which typically involves the initial growth of metal "islands" on the substrate surface to be coated with the tungsten. When tungsten is deposited on substrate materials such as silicon and titanium-tungsten alloys, it nucleates relatively rapidly. Two major factors influencing the reactive adsorption are believed to be substrate surface composition and temperature. For deposition on substrates having surfaces of silicon, for example, nucleation of tungsten by the hydrogen reduction process occurs readily at temperatures as low as 300.degree. C.
Deposition of tungsten on dielectric surfaces, such as titanium nitride (TiN), is more difficult when using the hydrogen reduction process. Researchers investigating deposition of tungsten on TiN have noted that for a deposition temperature of 450.degree. C., nucleation is inhibited onto TiN surfaces as compared to deposition onto surfaces of aluminum or silicon. The same researchers investigated the initial surface of the TiN and found it to be oxidized to some extent; Rana, V. V. S., et al., Tungsten and Other Refractory Metals for VLSI Applications, Vol II, pages 187-195, (Materials Research Society, E. K. Broadbent, ed. 1987).
As a result of the slow deposition of tungsten onto TiN surfaces with the hydrogen reduction process, deposition of tungsten onto titanium nitride for semiconductor applications now frequently resort to a silane (SiH.sub.4) reduction process. The chemistry of the silane reduction reaction provides a higher heat of reaction than does the hydrogen reduction process and better facilitates tungsten film nucleation on titanium nitride. Films deposited by the silane reduction process, however, tend to have higher sheet resistance than those deposited with the hydrogen reduction process. Accordingly, commercial processes often employ a two stage process: first, a nucleation layer is deposited with the silane reduction process, and second, a final tungsten layer is deposited over the nucleation layer with the hydrogen reduction process.
U.S. Pat. No. 5,342,652, issued to Foster et al., discloses a method where tungsten nucleation is brought about on a TiN coated substrate, which might have been exposed to atmosphere, by using particularly controlled sequences of attaining reaction pressure and temperature. Such nucleation method is also proposed for use in a cluster tool CVD module where a TiN film has been deposited by CVD in one chamber of the tool, and then, without breaking the vacuum of the apparatus or exposing TiN coated substrate to atmosphere, the TiN coated substrate is coated following nucleation by the process, either in the same chamber of the tool or another module of the tool after being transferred through a transport module, while maintaining an inert low pressure atmosphere therein, into the second CVD module in which the nucleation process is brought about and the tungsten film is applied.
While the process disclosed in U.S. Pat. No. 5,342,652 prevents an oxide layer from forming on the TiN film formed by CVD and facilitates nucleation of the deposited tungsten, it has not gained widespread acceptance in commercial applications, where the most common and preferred technology for formation of TiN layers is through physical vapor deposition or sputter coating. The vacuum requirements for physical vapor deposition of TiN differ significantly from the requirements for CVD of tungsten. Accordingly, it is difficult and expensive to combine a physical vapor deposition apparatus for forming TiN films with a CVD apparatus for forming tungsten films so that the two processes can be carried out without intervening exposure to atmosphere. Furthermore, while the process described in U.S. Pat. No. 5,342,652 improves tungsten nucleation, it has been found that greater improvement and more reliable nucleation of tungsten is desired under certain process practical conditions, such as with high concentrations of WF.sub.6 and H.sub.2, for example with greater than 100 Torr of H.sub.2, and with wafer temperatures less than about 435.degree. C.
Therefore, in a manufacturing environment, TiN films are typically formed on a substrate in a physical vapor deposition apparatus by sputtering a titanium metal target with reactive nitrogen gas. Formation of a tungsten layer over the sputter coated TiN film is then most efficiently achieved by removing the TiN coated substrate from the physical vapor deposition apparatus, at which time it is almost unavoidably exposed to air, and then placing the TiN coated substrate in the CVD apparatus where the tungsten film is deposited. As a result, the two step tungsten CVD process that includes a silanation step is still commonly used to maintain an acceptable tungsten nucleation and deposition rate.
It is advantageous to eliminate the requirement for the silanation step from tungsten CVD of TiN substrates for several reasons. First, the use of silane in tungsten CVD causes silicon to be incorporated into the tungsten films resulting in increased sheet resistivity of the deposited tungsten film. Second the hydrogen reduction process tends to conform less to deep recesses in integrated circuit structures. Additionally, the use of silane in a production environment can be problematic. Even a temporary leak in a silane gas line can result in a severe problem with particulate contamination. Further, silane is a hazardous substance and its presence in the manufacturing facility, where permitted by laws and regulations, is nonetheless undesirable. The substance is toxic, flammable, explosive and expensive to handle and maintain safely.
In view of the many problems associated with the two stage tungsten CVD process described above, it is advantageous to eliminate the requirement for the silane nucleation step while maintaining an acceptable tungsten nucleation and deposition rate. Additionally, due to the fact that CVD of the underlying TiN film has not gained widespread acceptance, a process is required that will allow rapid nucleation and deposition of tungsten on air exposed TiN films formed by physical vapor deposition methods.